This invention relates generally to a fluorescent lamp ballast and, more particularly, to a dimmable cold cathode fluorescent lamp (CCFL) ballast for liquid crystal display (LCD) backlighting of a laptop computer.
Efficiency, cost, and size are critical factors in the design of a CCFL ballast for LCD backlighting of a laptop computer. Conventional ballasts for LCD backlighting, such as ballasts sold by TDK Corporation of Tokyo, Japan, as part no. CXA-K05L-FS, include a buck converter and a current-fed self-oscillating push-pull inverter (also referred to as a Royer inverter). The overall efficiency of the combination of the buck stage and Royer inverter is inherently limited by the two power converter stages included therein. Additional power losses, inter alia, stem from the magnetizing inductance of the transformer within the Royer inverter serving as the resonant inductance. The typical efficiency of the buck stage combined with the Royer inverter is about 80%.
Another type of conventional ballast, such as part no. LXM1590/LXM1591 sold by Linfinity Microelectronics of Garden Grove, Calif., employs a half-bridge type inverter. The half-bridge type inverter is a more efficient ballast than the buck stage/push-pull type inverter combination. Similar to the push-pull type inverter, the half-bridge type inverter includes a transformer. The transformer in providing reactive power from its secondary winding to a ballasting capacitor in series with the lamp increases the circulating current. Real power losses from the increase in circulating current reduce the efficiency of the ballast. Alternatively, the transformer can be made larger in size to reduce winding resistance and thereby avoid the power losses resulting from the increase in circulating currents. Losses also arise from the equivalent series resistance (ESR) of a DC blocking capacitor. Typical efficiencies of a half-bridge type inverter are about 90%.
Accordingly, it is desirable to provide an improved ballast which is at least as efficient, less costly and smaller in size than a conventional ballast whether of the push-pull or half-bridge type.